The present invention relates to monitoring of x-ray exposures in imaging or inspection. It finds application in conjunction with diagnostic imaging apparatus, such as fluoroscopic imaging systems, and will be described with particular reference thereto. However, it should be appreciated that the invention will also find application in conjunction with other equipment where precise adjustable positioning of a radiation or visible light detection means is desirable.
Fluoroscopic imaging systems include a continuous or rapidly pulsed source of radiographic energy, such as that from an x-ray tube. The x-ray tube, when activated, propagates radiation through an object to be imaged, such as a human patient, onto a screen of fluorescent material. The object to be imaged is disposed in a gap between the x-ray source and the fluorescent screen.
X-ray radiation passing through the imaged object is attenuated according to the density of the material through which it has passed. Radiation passing through dense material, such as bone, will be attenuated more than radiation of similar energy passing through less dense materials, such as tissue. For uniformly intense radiation entering an object the radiation energy exiting the object is representative of the attenuation occurring within the object. Attenuated radiation impinging on the fluorescent screen is absorbed by fluorescent material thereon and converted into visible light image proportional to the radiation energy impinging at each point thereon. This conversion results in a two dimensional light image of the object represented by a plurality of different intensities of visible light. This light image can be visualized by the human eye or captured onto photographic film, which is generally more sensitive to light than to x-ray. The brightness of the fluorescent screen is sufficient to expose film placed in direct contact with the screen, but the light output is generally too low for direct diagnostic visualization, photographing with a camera, or viewing with a television camera. In many applications, a device is needed that will convert the x-rays into light and intensify, or increase, the brightness of the light. An image intensifier is such a device.
An image intensifier can be described as an evacuated glass bottle having a large bottom area as the input screen, and a small `cap` area as the output screen. The x-ray source side of the input screen has a fluorescent material disposed thereon for absorbing the incident x-rays and converting a portion thereof into a low level visible light image. The low level visible light is absorbed by at photo cathode layer disposed in the evacuated bottle. The absorption of light by the photo cathode layer results in the emission of low energy electrons into the evacuated bottle. An electrical energy source applies a bias voltage between the photo cathode and the output screen which accelerates the low energy electrons in the tube towards the output screen. A plurality of electrodes in the intensifier tube form an electrostatic lens which provides controlled acceleration and convergence of the electrons into a smaller image area on the output screen. The accelerated electrons, which are now at a high kinetic energy potential, converge on and strike the output screen which has a phosphor thereon. The convergence of the electrons on the output screen phosphor results in scintillations of light corresponding in intensity to the kinetic energy of the electron striking at each point on the output screen. The sum of the individual scintillations of light on the output screen results in an image on the output screen that is representative of the radiation at the input of the intensifier tube.
The input end of a viewing means, such as a video camera, is held in fixed relation to the output of the intensifier tube in order that the output image of the intensifier tube can be viewed by the camera input. The video camera is part of a closed circuit television system which provides a visual image on an image monitor that is representative of the radiation image detected at the input of the intensifier.
In fluoroscopy, the patient is exposed to a low intensity source of continuous or rapidly pulsed x-ray radiation so that the radiologist can dynamically view the operation of the internal body structure being imaged on the image monitor. In practice a balance is made between minimizing patient exposure to x-ray radiation and the need to provide sufficient radiation to produce a quality diagnostic image. Factors that influence the amount of radiation delivered to a patient in a particular imaging sequence are the path length the radiation will traverse within the patient and the attenuation of the radiation within patient structures being imaged. Preliminary selection of x-ray dose rate can be made on the basis of empirical data however, because path length and attenuation are patient dependent variables, the actual effect of x-ray dose selection is not known until the output image is viewed.
In some fluoroscopic imaging systems the x-ray source, image intensifier, video camera and related components are contained within a movable gantry structure which allows the fluoroscopic system to be dynamically positioned about the patient. Moreover, the patient couch can be moved, relative to the x-ray source and image intensifier, during imaging operations to optimize the image view. Adjustment of the gantry and/or patient couch during imaging results in a change in the image intensity change due to fluctuations in the radiation path length or different radiation attenuation characteristics in different portions of the patient. Because the viewed image is capable of dynamically changing it is desirable to provide a means to dynamically adjust the x-ray dose rate to maintain the same image quality regardless of changing conditions.
One way to assure consistent image quality is to measure the x-ray dose rate after the x-rays have passed through the object to be imaged. A way to accomplish this is to measure the intensity of the light image on the output screen of the image intensifier. Accordingly, a light sampling means is disposed to view the output screen of the image intensifier. One such sampling means is comprised of a pair of mirrors or prisms disposed on an adjustable mirror assembly, an opaque housing with a light input hole in the side thereof and a photo multiplier tube (PMT) disposed in the housing. The PMT is an externally biased vacuum tube device that produces an electrical output in response to light input on a collector array therein. The PMT is disposed in the housing such that light passing through the light input hole strikes the collector array. In operation a first support arm operatively positions one of the mirrors into the visible light path and a second support arm supports the other mirror. The support arms hold the mirrors such that they are aligned to reflect light propagating from the intensifier output screen into the light input hole wherein the PMT collector converts the incident light into an electrical signal equivalent of the light received thereby. The output of the PMT is connected to a controller which adjusts the source of radiation such that the light image on the output screen is maintained at a predetermined brightness level for diagnostic viewing.
In application, the radiation passing through the object to be imaged passes through more areas of the object in the area of interest. These other areas routinely attenuate the radiation differently than the area of interest thereby resulting in different levels of brightness on the output screen. Accordingly, it is desirable to have the sampling means sample the light from a select area of the output screen corresponding to the area of interest in the imaged object. Adjustment of the above described sampling means requires the operator to insert an x-ray mask in the radiation beam path between the radiation source and the intensifier input screen. The mask is a sheet of x-ray opaque and x-ray transmissive pattern portions which causes a known x-ray pattern to be disposed on the input screen when the x-ray source is engaged. The x-ray pattern striking the input screen results in a representative light pattern on the output screen of the intensifier tube. To adjust the sampling means such that it monitors the light from a select portion of the intensifier output screen, it is necessary to engage the x-ray source with the x-ray mask in position. With the known pattern on the output screen the position of the mirror(s) is manually adjusted until the PMT receives light from the select portion of the output screen pattern. Thereafter, the x-ray source is disengaged and the x-ray mask is removed from the beam path.
One problem with the prior art devices is that the x-ray source must be engaged in order to adjust the viewing positions of the mirror(s) to a select portion of the image intensifier output screen.
The present invention contemplates a new and improved photo detection means which utilizes inverse projection of a test pattern to adjust the optical alignment of the detection means thereby avoiding the need to engage the radiation source during alignment.